Figure 7.  Sodium-coupled accumulation.  As will be covered in Professor Wright’s lectures, most eucaryotic plasma membrane symporters translocate sub­strate with sodium, and the enegetics of sodium coupled transport is precisely the same as that described for proton coupled symport (i.e., there is an electrical compo­nent and a chemical gradient of sodium that act as the total driving force, the electro­chemical sodium gradient).  Although most ion-coupled transporters in E. coli utilize protons as the symported ion, in some cases, sodium is used. 

One bacterial ion-coupled symporter that uses sodium as the symported cation is the melibiose permease when the non-metabolized analogue thiomethylgalactoside (TMG) is the substrate.  As always, protons are pumped out via the respiratory chain (upper right) or the F₁F_o-ATP synthase (not shown), generating a (interior negative and alkaline).  Part of the is converted into a sodium gradient (low sodium and negative inside) by the sodium/proton antiporter, thereby providing the driving force for TMG/sodium symport.